Galaxy Clusters Throttled by Dark Energy

A decade ago, the study of distant supernovae led to the discovery of dark energy, which is accelerating the expansion of the universe. Now, researchers have confirmed the existence of this mysterious, repulsive force using a new, independent line of evidence. The findings provide new evidence for Einstein's theory of general relativity and add support to the idea that dark energy is an intrinsic and immutable property of vacuum.

Astrophysicist Alexey Vikhlinin and his colleagues at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, charted galaxy clusters as they grew over billions of years. These massive conglomerates of dust and gas are bound together by gravity in the near and distant universe. Analyzing x-ray images from NASA's Chandra X-ray Observatory Center, they found that the growth of these structures began to slow down 5.5 billion years ago, the team reported at a press conference today.

That period was in fact a crucial moment in the tug of war between the outward push of dark energy and the inward pull of gravity. In other words, the universe had stretched so much by this point that its dark energy made it difficult for galaxy clusters to pull matter in from far away. Not only did existing galaxy clusters slow their growth as a result, the rate at which new clusters were formed also declined.

"What we saw was an unmistakable effect of dark energy," says Vikhlinin, whose team is publishing the results in two papers in the 10 February 2009 issue of The Astrophysical Journal. The timing of dark energy's dampening effect on cluster growth coincides with findings by supernovae researchers showing that the universe's expansion had been decelerating before beginning to accelerate 5.5 billion years ago.

David Spergel, a theoretical astrophysicist at Princeton University, says the consistency between results from the two techniques "is a triumph for [Einstein's theory of] general relativity," which describes gravity as a property of the geometry of space and time. The finding helps rule out some alternative explanations for the universe's acceleration and takes researchers a step closer to understanding dark energy, he says.